For the production of high pixel density OLED (Organic Light Emitting Diode), a final metal mask (FFM) is commonly used as the mask in the vapor deposition of organic luminophore (three primary colors) within the pixel, which is quite thin and has a low coefficient of thermal expansion.
FFM used in OLED is commonly produced with invar steel through chemical etching The specific process is to coat the surface of invar steel with photoresist. Then, patterns of FFM will be transferred to the light sensitive layer by exposure. The final procedure will be completed by developing and chemical etching.
The normal thickness of FFM is only 30 to 200 μm. Due to its limited ductility and low thickness, it is extremely difficult to keep the FFM attached to the surface of a substrate (glass or flexible plate) with organic luminous sub-pixels and ensure high positional accuracy. It requires welding FFM to metal frames by laser. During the whole process, FFM is easily damaged or shifted because of uneven forces applied to it or thermal effect, etc. Besides, FFM should be regularly cleaned to avoid pattern deficiency caused by residual materials used in the manufacturing of OLED. Cleaning is prone to cause the damage of some pattern, so it is necessary to replace FFM every two months. All the above reasons make the use of FFM very expensive during the manufacture of OLED.